This invention relates to a circuit for judging the existence of television (TV) image signals such as applied to NTSC system or PAL system TVs.
FIG. 3 is a conceptional block diagram showing a construction of essential parts of the conventional circuit for judging the existence of television image signals. In the same drawing, numeral 1 is the conventional circuit for judging the existence of television image signals, numeral 2 is an input terminal of composite synchronous signals, and numeral 3 shows a synchronous counter for counting edges of the composite synchronous signals. Also, numeral 8 is a counted number analyzer for receiving the counted synchronous signals from the synchronous counter for analyzing counted values and for outputting a judged result, and numeral 9 is an output terminal for generating the judged result.
The operation of the conventional circuit 1 is next described as follows.
When normal image signals are obtained as shown in FIG. 4A, the edges of the horizontal synchronous signals of the normal image signals are detected, then the composite synchronous signals as shown in FIG. 4B are generated and supplied into the input terminal 2 of the image judging circuit 1. The synchronous counter 3 then counts the number of each of the rising edges of the composite synchronous signals, and the counted number analyzer 8 judges that the normal image signals are being transmitted based on the number of counted edges, and finally the judged results are output from the terminal 9.
In receiving the television broadcasting signals, as the conventional image signal judging circuit 1 is constructed as described above, it becomes impossible to see the television images because the synchronous counter detects the noise caused by the distortion of the image signals themselves together with the normal image signals, despite normally receiving the image signals, and the counter number analyzer 8 misjudges the noise as the correct normal signals.
Namely, noise caused by the aforementioned distortion is apt to be generated immediately after the horizontal synchronous signals, and practically as shown in FIGS. 5A and 5B, noise 10 is generated.
In the television image signals including the noise 10 shown in FIG. 5A, composite synchronous signals generated by detecting the edge of horizontal synchronous signals of the image signals, includes noise 10 shown in FIG. 5B, and accordingly the counted number detected by the synchronous counter 3 becomes two times the number of normal image signals, and the counted number analyzer 8 issues a signal indicating the nonexistence of image signals. Namely, there was a problem that the analyzer misjudges that image signals cannot be detected in spite of receiving normal image signals.
This invention was developed to overcome the above-mentioned drawbacks, and the object thereof is to provide an image signal judging circuit enabling an increase in the reliability thereof.